This project is concerned with a comparative analysis of ionic current channels in nerve and heart cell membranes and the relationship of these channels to electrical activity, with a particular emphasis on potassium ion channels in both preparations and the effects of various ionic blockers on these channels. During the past year the primary experimental preparations which have been used are squid giant axons and chick embryonic heart cells. This work has focused recently on the Delayed Rectifier Potassium Channel, IK, in chick heart cells. A major finding has been that measurements of this component in single cells using the suction pipette voltage clamp technique are comparable in every way to our earlier measurements of IK in aggregates of cells using the two-microelectrode voltage clamp technique. This result challenges a prevailing view concerning the role of potassium ion accumulation in the extracellular spaces of cardiac tissue. Specifically, the loading of these spaces by potassium ions is believed to play a role in generating Cardiac Dysrhythmias. Our results suggest that this mechanism may not be as significant as is commonly believed.